1. Field of the Invention
The present invention relates to a semiconductor photonic element such as a semiconductor laser having a multilayer current-constricting structure, a method of fabricating the element, and a semiconductor photonic device using the element.
2. Description of the Prior Art
In recent years, to realize semiconductor lasers having excellent characteristics such as low threshold current, high efficiency, and high output, there has been the strong need to form a high-performance current-constricting structure thereby increasing the injection efficiency of a driving current. To meet the need, conventionally, various improvements have ever been made for the current-constricting structure.
FIG. 1 shows a partial cross-section of a prior-art semiconductor laser of this sort, in which a conventional current-constricting structure is used.
As shown in FIG. 1, the prior-art semiconductor laser 100 comprises an n-type InP substrate 101, and a mesa structure 140 formed on an upper main surface of the substrate 101, and a current-constricting structure 150 formed on the surface of the substrate 101 at each side of the mesa structure 140. The mesa structure 140 includes an n-type InP cladding layer 102 formed on the surface of the substrate 101, a semiconductor active layer 103 formed on the layer 102, and a p-type InP cladding layer 104 formed on the layer 103. The current-constricting structure 150 includes a p-type InP current-blocking layer 105 formed on the surface of the substrate 101, and an n-type InP current-blocking layer 106 formed on the layer 105.
A p-type InP burying layer 107 is formed to cover the mesa structure 140 and the current-constricting structure 150. A p-type InGaAs contact layer 108 is formed on the layer 107. A p-side electrode 109 is formed on the layer 108. An n-side electrode 110 is formed on a lower main surface of the substrate 101.
As seen from FIG. 1, the current-constricting structure 150 is formed by the p- and n-InP current-blocking layers 105 and 106, which intervene between the n-type InP substrate 101 and the p-type InP burying layer 107. Thus, the prior-art semiconductor laser of FIG. 1 has a pnpn structure (i.e., the thyristor structure), which causes the following problem.
It is supposed that a leakage current flows from the point A in the burying layer 107 to the point B in the substrate 101. In this case, this leakage current serves as a gate current of the thyristor structure and as a result, a leakage current (which serves as an anode current of the thyristor structure) tends to flow from the point C in the burying layer 107 to the point D in the substrate 101, as shown in FIG. 1. Accordingly, the undesired turn-on tends to occur in the thyristor structure, thereby losing the current-constricting function of the current-constricting structure 150. When the prior-art laser 100 operates in a high-temperature and/or high-output condition where the leakage current becomes large, the undesired turn-on often occurs in particular.
As described above, although the current-constricting structure 150 in the prior-art semiconductor laser 100 of FIG. 1 is effective for lowering the threshold current, it has the above-identified problem due to the undesired turn-on in the thyristor structure.
FIG. 2 shows a partial cross-section of a prior-art semiconductor laser of this sort using an improved current-constricting structure for suppressing the above-described undesired turn-on problem.
The prior-art semiconductor laser 200 of FIG. 2 has the same configuration as that of the prior-art semiconductor laser 100 of FIG. 1 except that a current-constricting structure 250 including two mesa structures 251 is used instead of the current-constricting structure 150. Therefore, the explanation about the same configuration is omitted here for simplification of description by attaching the same reference symbols as those used in FIG. 1 to corresponding elements in FIG. 2.
Each of the mesa structures 251 in the current-constricting structure 250 includes an n-type InP layer 202 formed on the upper main surface of the substrate 101, an InGaAsP recombination layer 212 formed on the layer 202, and a p-type InP layer 204 formed on the layer 212. The structures 251 are located under the p-type InP blocking layer 105 at each side of the mesa structure 140 having the active layer 103.
The recombination layer 212 serves to cancel the leakage current (which serves as the anode current of the thyristor) due to carrier recombination, thereby decreasing the current gain factor of the thyristor structure. Thus, the unwanted turn-on of the thyristor structure can be suppressed.
Another improved current-constricting structure using a dielectric layer is disclosed in the paper written by N. Iwai et al., Electronics Letters, Vol. 34, No. 14, pp. 1427-1428, Jul. 9, 1998. FIGS. 3A and 3B show partial cross-sections showing the fabrication method of a prior-art semiconductor laser of this sort, which has the improved current-constricting structure disclosed in this paper.
First, as shown in FIG. 3A, a p-type InP layer 317 with a thickness of 50 nm is formed on an upper main surface of a p-type InP substrate 316. Then, a p-type InAlAs layer 314 with a thickness of 50 nm is formed on the layer 317, and a p-type InP layer 321 with a thickness of 100 nm is formed on the layer 314. A Multiple Quantum Well (MQW) active layer 303, which is formed by alternately stacking InGaAsP barrier sublayers and InGaAsP well sublayers, is formed on the layer 321. An n-type InP layer 315 is formed on the layer 303. An n-type InGaAs contact layer 318 is formed on the layer 315. These layers 317, 314, 321, 303, 315, and 318 are formed by using the Metal Organic Vapor Phase Epitaxy (MOVPE) technique.
Subsequently, channels 320 are formed to reach the underlying InP substrate 316 through the layers 318, 315, 303, 321, 314, and 317, thereby forming a mesa structure 328, as shown in FIG. 3A. For example, the pitch of the channels 320 (i.e., the width of the mesa structure 328) is set as approximately 10 .mu.m.
Following this, the substrate 316 having the structure of FIG. 3A is placed in an oxidation furnace, thereby selectively oxidizing the p-type InAlAs layer 314 to form a dielectric layer 319 while only the strip-shaped middle mart of the layer 314 is not oxidized, as shown in FIG. 3B. The oxidation of the layer 314 begins at its ends exposed to the channels 320, and progresses laterally toward the center of the layer 314. The middle part of the layer 314 serves as the current injection region through which a driving current is injected. The oxidation period is adjusted so that the remaining middle part of the layer 314 has a width of approximately 4.6 .mu.m. For example, it is set as 150 minutes.
Furthermore, a silicon dioxide (SiO.sub.2) layer 313 is formed on the n-type InGaAs contact layer 318 and the inner walls of the channels 320. A strip-shaped window 313a is formed in the layer 313 to be overlapped with the remaining InAlAs layer 314. An n-side electrode 310 is formed on the SiO.sub.2 layer 313. A p-side electrodes 311 is formed on the lower main surface of the substrate 316. Thus, the prior-art semiconductor laser 300 is fabricated, as shown in FIG. 3B.
In the prior-art semiconductor Laser 300 shown in FIG. 38, the dielectric layer 319 serves as a current-blocking layer. When the length of the resonator (i.e., the optical waveguide) is set as 300 .mu.m and high-reflectance coating with a 96% reflectance is applied to the rear end of the waveguide, the obtainable threshold current for continuous oscillation at 25.degree. C. is 18 mA and the obtainable slope efficiency is 0.55 W/A.
A further improved current-constricting structure using a dielectric layer is disclosed in the 16th Semiconductor Laser International Conference Digest, pp. 157-158, 1998, which was reported by Wang Zhi Jie et al. FIGS. 4A and 4B show partial cross-sections showing the fabrication method of a prior-art semiconductor laser 400 of this sort, which includes the improved current-constricting structure disclosed in this digest.
As shown in FIG. 4A, first, an n-type InP cladding layer 402 is formed on the whole upper main surface of an n-type InP substrate 416. Then, a MQW active layer 403, which is formed by alternately stacking InGaAsP barrier sublayers and InGaAsP well sublayers, is formed on the layer 402. Ap-type InP cladding layer 404 is formed on the layer 403. These layers 402, 403, and 404 are formed by using the MOVPE technique.
Next, the stacked layers 402, 403, and 404 are selectively etched to form a strip-shaped mesa structure 440 on the surface of the substrate 416. For example, the mesa structure 440 is 2 .mu.m in width and 0.6 .mu.m in height.
Thereafter, a p-type InP layer 415 with a thickness of 0.65 .mu.m is formed on the main surface of the substrate 416 to cover the mesa structure 440. Ap-type InAlAs layer 414 with a thickness of 100 nm, which is used for making a current-blocking dielectric layer 419, is formed on the layer 415. A p-type InP burying layer 407 is formed on the layer 414. A p-type InGaAs contact layer 408 is formed on the layer 407.
Subsequently, channels 420 are formed to reach the underlying InP substrate 416 through the layers 408, 407, 414, and 415, forming a strip-shaped mesa structure 428, as shown in FIG. 4A. Thus, the so-called double mesa structure is formed on the substrate 2 by the mesa structures 440 and 428. For example, the pitch of the channels 420 (i.e., the width of the structure 428) is set as approximately 10 .mu.m.
Following this, the substrate 416 having the structure of FIG. 4A is placed in an oxidation furnace, thereby selectively oxidizing the p-type InAlAs layer 414 to form the dielectric layer 419 while only the strip-shaped middle mart of the layer 414 is not oxidized, as shown in FIG. 4B. The oxidation of the layer 414 begins at its ends exposed to the channels 420, and progresses laterally toward the center of the layer 414. The unoxidized middle part of the layer 414, which extends along the mesa structure 440, serves as the current injection region through which a driving current is injected. The oxidation period is adjusted so that the remaining middle part of the layer 414 has a width of approximately 5 .mu.m. For example, it is set as one hour.
Furthermore, a SiO.sub.2 layer 413 is formed on the contact layer 408 and the inner walls of the channels 420. A window 413a, which has a shape corresponding to that of the mesa structure 440, is formed in the layer 413 to be overlapped with the remaining InAlAs layer 414 and the mesa structure 440. A p-side electrode 410 is formed on the SiO.sub.2 layer 413 to be contacted with the contact layer 408 through the window 413a. An n-side electrode 411 is formed on the lower main surface of the substrate 416. Thus, the prior-art semiconductor laser 400 is fabricated, as shown in FIG. 4B.
In the prior-art semiconductor laser 400 shown in FIG. 4B, the dielectric layer 419 serves as a current-blocking layer.
With the above-explained prior-art semiconductor lasers 200, 300, and 400, the unwanted turn-on of the thyristor structure in the prior-art semiconductor laser 100 of FIG. 1 can be suppressed or eliminated. However, they have other problems described below.
With the prior-art semiconductor laser 200 shown in FIG. 2, a comparatively large current tends to flow through the recombination layer 212 along the path from the point A in the layer 107 to the point B in the substrate 101 even when the driving or injection current is low. Thus, a problem that the threshold current is unable or difficult to be lowered as desired will occur.
With the prior-art semiconductor laser 300 shown in FIG. 3B, the strip-shaped optical waveguide formed under the window 313a of the SiO.sub.2 layer does not have the buried mesa structure, which is of the ridge type. Thus, effective current constriction is unable to be realized, resulting in the same problem as that identified in the prior-art semiconductor laser 200.
Moreover, because of a crystallographic reason, the dielectric layer 319 given by the oxidation of the InAlAs layer 314 is difficult to be located at a short distance of 0.1 .mu.m or less from the MQW active layer 303. Thus, a comparatively large leakage current tends to flow through the gap between the dielectric layer 319 and the active layer 303. As a result, there arises a problem that the laser efficiency cannot be raised in the high-output operation condition, although the undesired turn-on of the thyristor structure can be prevented.
Additionally, the oxidation length of the p-type InAlAs layer 314 that determines the width of the current injection region is controlled only by changing the oxidation period of the layer 314. Thus, there arises a problem that the formation of the dielectric layer 319 through the oxidation process does not have satisfactory reproducibility and satisfactory in-plane uniformity, and that the width of the current injection region fluctuates along its axial direction to result in degradation in laser characteristics.
With the prior-art semiconductor laser 400 shown in FIG. 4B, the optical waveguide has the buried mesa structure 440 and therefore, the path for the leakage current can be limited or narrowed compared with the prior-art laser 300 shown in FIG. 3B. However, the width of the current injection region is unable to be narrowed as desired, because the formation process of the current injection region is performed by controlling the oxidation period of the InAlAs layer 414 similar to the prior-art laser 300. Accordingly, a comparatively large leakage current tends to flow, thereby causing a problem that the threshold current is unable or difficult to be decreased.
There is another problem that the formation of the dielectric layer 419 through the oxidation process does not have satisfactory reproducibility and satisfactory in-plane uniformity, similar to the prior-art laser 300.
Furthermore, as seen from FIG. 4B, the unoxidized InAlAs layer 414, the resistance of which is difficult to be lowered, exists over the active layer 403. Thus, there is a further problem that the current injection efficiency is degraded remarkably.
In summary, the prior-art semiconductor laser 100 of FIG. 1 has the problem that the unwanted turn-on of the thyristor structure tends to occur although the threshold current can be lowered. This means that the laser 100 has unsatisfactory characteristics in the high-temperature and/or high-output operation condition.
Each of the prior-art semiconductor lasers 200, 300, and 400 of FIGS. 2, 3B, and 4B can solve the unwanted turn-on problem of the prior-art semiconductor laser 100. However, it has the problem that the threshold current is unable or difficult to be lowered. In other words, none of them realizes a current-constricting structure that satisfies both the low threshold current and the satisfactory characteristics at high-temperature and/or high-output operating conditions.